Isolation and characterization of temperature sensitive mutants and defective mutants of avian sarcoma virus will be made. Interaction of these mutants with the endogenous virus will be studied. Homology among avian RNA tumor virus RNA will be studied by molecular hybridization, that inclues the endogenous virus and its recombinants. Analysis of viral glycoprotein in cells by radio immunoassay will be made. Genetic cross of chickens will be made to determine the mode of transmission of the characteristic contributing the high level of helper activity in normal chicken cells.